Evidence for glycine, GABAA, and GABAB receptors on rabbit OFF-alpha ganglion cells

Inhibitory synaptic transmission via GABA and glycine receptors plays a crucial role in shaping the excitatory response of neurons in the retina. Whole-cell recordings were obtained from ganglion cells in the intact rabbit eyecup preparation to correlate GABA- and glycine-activated currents with the presence of their specific receptors on morphologically identified a ganglion cells. Alpha ganglion cells were chosen based upon their large somata when viewing the retinal surface, and responses to light and dark spots were used to identify OFF-alpha ganglion cells. Light responses were abolished by superfusion of Ringer's containing cobalt to synaptically isolate the cell by blocking all Ca2+-mediated transmitter release. Pressure pulses of GABA and glycine were delivered to an area that encompassed the dendritic field while receptor antagonists were applied through superfusion to characterize the direct inhibition onto the ganglion cell. Physiological results indicated that OFF-a cells did not have any GABA(C) receptor-activated currents, but did express currents mediated by ionotropic GABA(A) receptors and metabotropic GABA(B) receptors that were blocked by their specific antagonists bicuculline and CGP55845, respectively. The amplitudes of strychnine-sensitive glycine-activated currents were always larger than the currents elicited by GABA. Confocal optical sections of physiologically identified, sulforhodamine B-stained cells displayed the localization of glycine and GABA(A) receptor subunit labeling dispersed over the stained dendrites. Although scant labeling of GABA(B) receptors was found on the more distal dendrites, the majority of these receptors were congregated at the soma and on the proximal dendrites close to the soma. No GABA(C) receptor immunoreactivity was found anywhere on these cells. Therefore, the immunocytochemical results corroborated the physiological evidence demonstrating that OFF-a ganglion cells in the rabbit retina express functional GABA(A), GABA(B), and glycine receptors, but no GABA(C) receptors.